Venus orbits the Sun within Earth's orbit, so it occasionally happens that as seen from Earth, the disk of Venus passes across the Sun. It appears as a diminutive black spot, barely 1/30th the diameter of the Sun. With the right atmospheric conditions to soften the intense sunlight, an unobstructed horizon, and enough advance warning, a keen eye can spot the transit at sunrise or sunset.
To avoid blindness, never observe the Sun without proper eye protection.
More information at What is the Transit Venus.
Library of Congress, I Hear America Sing, 2004